passing path to checkout to update() causes data file to be destroyed

[sascha_silbe]

If we call datastore.get() and pass the file name back into datastore.update() for the same entry with transfer_ownership set to False, the data file will get destroyed.
This is because source and destination file are identical (hard links) and we overwrite the destination file in AsyncCopy.start() without unlinking it first.

[erikos]

Secondary information:  http://lists.sugarlabs.org/archive/sugar-devel/2011-March/030264.html

[erikos]

Testcase:

- open Record and take a photo

- switch to the Journal and wait for the picture entry to appear

- go back to Record

- rename the photo

- close Record

- open the photo in Imageviewer / open the Record session and show the picture

[erikos]

 http://git.sugarlabs.org/sugar-datastore/mainline/commit/ad9244e31184415e73be075e9ace053fca97d2f5

[erikos]

Did land, closing. QA happens on  http://dev.laptop.org/ticket/10957